1. Field of the Invention
The present invention relates to a method of fabricating a lead frame for a semiconductor device fabricated by resin-sealing a semiconductor chip and a method of fabricating a semiconductor device using the same, and such a lead frame and a semiconductor device using the same.
2. Description of Related Art
In order to high-density mount a semiconductor device on a wiring substrate, a package for high-density mounting allowing surface-mounting on the wiring substrate by eliminating extension of a lead from a molding resin package and exposing a lead (a terminal portion electrically connected to a semiconductor chip) of a lead frame to a lower surface of the package has been conventionally used. Known as such a package for high-density mounting have been leadless packages such as QFN (Quad Flat Non-leaded Package) or SON (Small Outlined Non-leaded Package).
In the packages in these forms, a lower surface of the lead sealed with molding resin, together with the semiconductor chip, is exposed to the lower surface of the package, so that the lead easily slips off the molding resin. Therefore, the lead can be prevented from slipping off by forming the lead into a reverse tapered shape and forming a step on a side surface of the lead.
In order to form the lead having such a cross-sectional shape, the lead frame has been conventionally processed by etching. However, a long time is required for the processing. Therefore, the fabrication of the lead frame using a precision press metal mold has been recently proposed, as disclosed in U.S. Pat. No. 6,664,133.
In the method disclosed in the U.S. Patent, the lead frame is fabricated by subjecting a metal plate serving as a material for the lead frame to punching processing and pressing processing using a punch from the side of a lower surface of the lead frame, and stepped shapes for slipping prevention are respectively formed at a front end and on a side surface of the lead.
More specifically, in the method disclosed in the US patent, the center of the lead is made wide, and a side edge of the center is subjected to pressing processing using a punch from the side of a lower surface of the lead, thereby forming a two-step shape for slipping prevention. For the purpose of preventing a burr from occurring in the case of pressing processing from the side of the lower surface, a constricted part is formed on a side surface between the center and the base of the lead.
In the method disclosed in the US patent, the shape of the lower surface of the lead obtained by the punching processing is lost in performing the pressing processing from the side of the lower surface. Therefore, the lower surface of the lead for making electrical connection to the exterior may not have a desired shape (e.g., a linear shape).
In order to prevent the metal burr occurring in the case of frame processing, the constricted part must be formed between the center and the base of the lead, so that the shape of the lead first formed by the punching processing is complicated. Consequently, required as a metal mold (a punch) for the punching processing is one which is subjected to complicated processing.
In the prior art disclosed in the US patent, therefore, a lot of metal molds having a precise and complicated structure are required, so that the fabrication costs of the metal molds pile up. As a result, the fabrication cost of the lead frame and therefore, the fabrication cost of the semiconductor device rises.
On the other hand, in the above-mentioned semiconductor package, a package (e.g., HQFN: Heat Sinked Quad Flat Non-leaded Package) having a structure for exposing an island (a chip support having a semiconductor chip carried thereon) of a lead frame on a lower surface of the package has been put to practical use in order to enhance heat dissipating properties.
In the package having such a structure, a lower surface of the island, together with the semiconductor chip, sealed with molding resin is exposed to the lower surface of the package, so that the island easily slips off the molding resin. Therefore, an end surface of the island is made to have a reverse tapered shape, or the island is formed with a step to prevent the island from slipping off.
The island having such a cross-sectional shape has been formed by etching, as disclosed in US Patent Application Publication No. 2002/0096790A1, for example.
Since running cost is required to process the end surface of the island by etching, however, the lead frame cannot be fabricated at low cost.
In the prior art disclosed in the US Patent Application Publication, in order to expose both the lead and the island from the lower surface of the package, they have been arranged so as to be flush with each other before resin sealing. When the lead is interposed between upper and lower metal molds to supply molding resin to a cavity between the metal molds, a pressing force from the upper metal mold is not exerted on the island, so that the resin detours toward the lower surface of the island. Therefore, the lower surface of the island is insufficiently exposed, thereby not obtaining a desired cooling effect.